Rotating disc memory storage devices are used in conjunction with digital computers to magnetically store digital information on a non-volatile basis. A typical device of this type includes a spindle rotated by a drive motor mechanism, one or more recording discs attached to the spindle, either permanently or removably, for rotation therewith at a relatively precise constant speed, and an electromechanical read/write assembly for enabling information to be written onto and read from the disc recording surfaces. Each read/write assembly includes a number of transducers for reading and writing data magnetically from and to the individual disc recording surfaces, and a transducer motion translation mechanism, typically operated by a stepper motor, to effect radial motion of the transducers across the recording surface of each disc. The stepper motor is typically driven by control circuitry that receives electrical position signals in digital form from the associated computer, and converts this position information into mechanical motion of the transducer heads.
Each annular recording surface of a disc is usually arranged in the form of concentric circular tracks divided in the circumferential direction into track sectors, in order to enable access locations to be accurately specified by the associated computer for rapid and accurate information storage and retrieval.
Recent trends in the development of rotating disc memory storage devices have been toward reduction in the physical size of the system without sacrificing, and in many cases increasing, the storage capacity of each disc. These trends have been especially evident in disc drives designed for use in small business computers and personal computers. Such disc drives employ 51/4" or 31/2" fixed rigid discs, removable flexible (floppy) discs or a combination of both. Due to the relatively small surface area available for information storage on such discs, many efforts have been made to maximize the amount of information that can be accurately stored on such discs. These efforts have included a wide variety of specially designed recording techniques, read/write transducers with increasingly narrow heads (to reduce trace width) and disc recording layers with improved magnetic recording properties and finer surface smoothness.
To obtain the maximum storage capacity for a fixed cost, it is desirable that each disc contain the maximum number of bits and tracks per inch. As the track density increases, however, it becomes increasingly difficult to repeatably precisely position the heads. Therefore, it happens that from time to data integrity is lost, by virtue of the fact that the head was not properly positioned over the track during the writing operation, or that it is not now precisely positioned over the track for the read operation. Once such data integrity has been lost in a stepper motor position control system, a method of "off-track" data recovery must be employed.